1. Field of the Invention
This invention relates to a continuous, one-step, solvent-free process for the high yield manufacture of polyisocyanurate compounds containing blocked, pendant isocyanate groups. This invention also relates to the compounds which may be obtained according to the process and to the use of these compounds as curing agents, in particular as curing agents in powder coating compositions.
As used herein, the terms polyisocyanate, polyisocyanurate and solvent-free process are defined as follows:
(a) Polyisocyanate. Polyisocyanate means any monomeric species containing two or more isocyanate groups bound to an alkyl, cycloalkyl or aromatic moiety.
(b) Polvisocvanurate. Polyisocyanurate means any polyisocyanate which has been cyclo-polymerized to form a single ##STR1## ring system (x=2, 3 or 4) which contains two or more pendant isocyanate groups; in particular, a polyisocyanate that has been polymerized to form the trimeric ring system ##STR2## containing two or more pendant isocyanate groups.
(c) Solvent-free Process. Solvent-free process refers to a process or method of manufacture in which the polyisocyanate and the blocking agent are used in stoichiometric amounts, where the reaction has been carried out to substantial completion, and where no non-reactive inorganic or organic medium has been employed during the reaction.
2. Description of the Prior Art
Powder coating compositions and techniques have conventionally been used in the provision of protective films, and powder coating technology is becoming increasingly important for both economic and environmental reasons. Powder coating technology includes fluidized bed sintering techniques (such as electrostatic fluidized bed coating) and spray coating methods (such as electrostatic powder coating) which may be used or adapted for use with the various types of powder coating compositions. The compositions themselves may desirably be adapted and formulated to be, initially, sufficiently fluid to be capable of forming a film on the substrate under the coating conditions. The film may be subsequently curable by further polymerization and/or crosslinking.
There are a number of processes for the production of polyisocyanurates, the most important being the process in which a six member ##STR3## ring having pendant unreacted --R--NCO functional groups is produced by cyclo-trimerizing a polyisocyanate monomer, such as toluene diisocynate or isophorone diisocyanate. A catalyst, selected for its ability to promote trimerization, is employed in these processes. When the desired polyisocyanurate is intended for use as a curing agent, a blocking agent may be added to the reaction mixture. The blocking agent reacts with the pendant isocyanate groups in a temperature reversible reaction to give a stable blocked polyisocyanurate product that can be used in powder coating formulations. The known processes, however, suffer from the drawback that either a non-reactive solvent must be used in the manufacturing process or that the reaction is run to completion so that the reactants can provide their own solvent. In either instance, costly and time consuming separation steps, such as filtration and drying, are required to isolate the blocked partial polyisocyanurate product. For example, U.S. Pat. No. 4,552,946 to Scholl et al states that when monomeric [poly] isocyanates are trimerized in the presence of a solvent, the reaction may be carried to the range of 50-70% completion. When the reaction is conducted without a solvent [non-reactive species], it is usually terminated after 10-25% completion; at which point reacted starting material is removed, usually in a thin film evaporator. Scholl et al, in effect, is using the polyisocyanate as a solvent. U.S. Pat. No. 4,454,317, to Disteldorf et al, in teaching trimerization of polyisocynates to polyisocyanurates, states a maximum conversion of 45% of starting material when the polyisocyanate is trimerized without a solvent. U.S. Pat. No. 3,919,218 to Schmitt et al, in teaching trimerization in the presence or absence of organic solvent, specifically teaches that the polyisocyanate itself can be used as a solvent. When the polyisocyanate is so used, the reaction is presumably stopped far short of completion because thin film separation of the starting material is specifically recommended by Schmitt et al.
In contrast to Scholl, Disteldorf and Schmitt, the present invention contemplates substantially complete conversion of the polyisocyanate reactant to a polyisocyanurate product. Further, unlike the references described above, the present invention does not require the product to be treated to remove the polymerization catalyst.
The production of blocked polyisocyanurates is taught by Gras et al in U.S. Pat. No. 3,313,876. In Gras et al, however, the production of the blocked polyisocyanurate is a multiple step process involving 50-80% starting material, polymerization, destruction and removal of the catalyst, and finally addition of the blocking agent. In contrast, according to the present invention the conversion from polyisocyanate to polyisocyanurate is substantially complete, no catalyst destruction or removal is required, and the blocking of the pendant isocyanate groups does not require a separate step.
A one-step synthesis of compounds containing blocked isocyanate groups has been taught by Panandiker et al in U.S. Pat. No. 4,055,551. However, Panandiker et al teaches the synthesis of blocked isocyanatopolyurethane (urethane: --C--O--C--N--C--) from a polyhydric alcohol, a diisocyanate and a blocking agent. In Panandiker et al, one of two isocyanate groups is reacted with an alcohol hydroxyl to form the urethane function and the other isocyanate group is blocked. Panandiker et al do not teach, nor can it be inferred, that in the absence of the polyhydric alcohol the diisocyanate would cyclo-trimerize to form the polyisocyanurate having a cyclic ##STR4## ring structure and pendant organoisocyanate groups. Contrarily, the prior art teaches that in the absence of any non-reactive solvent, complete polymerization should be avoided in order to prevent the formation of macromolecular multiple ring compounds.
Accordingly, there is a need for an improved method for manufacturing blocked polyisocyanurate compounds, and it is the object of this invention to provide such a method.